1. Field of the Invention
The present invention relates to a ball screw with spacers and more particularly, to a ball screw with spacers each of them being interposed between two adjacent balls so as to prevent balls from colliding and impacting with each other.
2. Description of the Prior Art
A ball screw unit is combination of a screw bolt and a nut assembly with a plurality of circulating balls amidst them. The outer surface of the screw bolt is formed into screw grooves having a semi circular cross section. The nut assembly consists of a nut body and accessories providing means for the balls to roll around. The nut body is hollow with its inner surface formed into screw grooves having a semi circular cross section and each groove is facing against its corresponding groove formed on the screw bolt. The plurality of balls contained between the screw bolt and the nut assembly are circulating along the grooves with the aid of the accessories such that the ball screw and the nut assembly is rotatable with each other.
As it is well known that the balls are mostly made of metallic material, noise and vibration is apt to occur as they roll along the screw grooves of the screw bolt and the nut assembly. In addition, mutual friction between adjacent balls is produced due to different moving direction between them. When the balls are meandering along the circulation passage, the different curvatures of the circulation passage and the screw grooves thereof cause change of advancing speed of the balls with the result of mutual collision and impact among them. Recently there is better material, for example, ceramic is used to form a lighter ball than a conventional steel ball so as to reduce impact energy, and instead, increase hardness and improve anti-abrasion and damping effect. Meanwhile, there is a big problem that average users are unable and unwilling to afford use of this new and expensive ball.
FIG. 7 is a schematic view showing a structure of a conventional ball screw for moderating friction and impact between the balls. As shown in FIG. 7, a plurality of balls 4 are rolling amidst screw grooves 11 of the screw bolt and screw grooves 21 of the nut assembly. A smaller spacer ball 6 is provided between two adjacent balls 4 so as to keep the two adjacent balls 4 rotating in the same direction thereby reducing frictional resistance between the balls and improving movility of the screw bolt. Generally, the ball 4 and the spacer ball 6 are made of the same material. Although the frictional resistance arising from the different rotational direction of the balls 4 in both grooves 11 and 21 is evaded, yet there still remains no answer to the problem as to mutual collision and impact of the ball 4 moving along in the circulation passage due to change of speed.
U.S. Pat. No. 4,505,522 provided a method to change accommodative numbers of balls for reducing the balls which enter or go out of the circulation passage. However, owing to complexity of the circulation passage pertaining to the ball screw mechanism, a simple geometrical figure is even found to be difficult to design in the case such a technology is applied. As a result, control of amount of balls for the screw bolt does not effectively utilized. If it is desired to well utilize the method disclosed by this cited invention, determination of a precise geometrical figure for the circulation passage should be made in the first priority, this further accompanies fabrication difficulty with the ball screw mechanism.
The U.S. Pat. No. 5,615,955 suggested use of materials with lubricating function such as oil containing plastics to form the spacer balls 6 shown in FIG. 7 to exhibit both anti-vibration property and lubrication effect. It is really with preferable flexibility to deformation and reduces noise arising from impact as well. However, it is regretful that the above advantages are counteracted by the fact that accommodative balls 4 which can effectively with stand load in the ball screw mechanism are reduced to half as many as those are originally contained therein due to large available spacing is occupied by the spacer balls 6 whose sizes are nearly equal to those of the balls 4. The strength of ball screw mechanism is therefore degraded.
The present invention has been made in order to eliminate the inconvenience inherent to the conventional techniques as mentioned above. Thus, an object of the present invention is aimed at solving the problem of noise and vibration arising from mutual collision and impact of balls which are rolling along in the screw grooves of both screw bolt and nut body and in the circulation passage due to difference of moving speed and force impressed each other.
It is another object of the present invention to prevent reducing amount of load carrying balls due to occupation of available space by bulky spacer balls.
These and other objects of the present invention are achieved by a smaller spacer to be interposed between two adjacent balls so as to minimize frictional resistance and impact force therebetween thereby reducing noise and vibration as small as possible. This spacer is designed into a cylindrical shaped body with two ends formed into inwardly concaved surfaces so as to match the shape of the balls thereby greatly reducing the space between two adjacent balls. As a result, the amount of balls accommodative in the ball screw unit is increased with enhanced mechanical strength.
In the present invention, the above mentioned cylindrical spacer does not roll in the circulation passage, in order to prevent the unrolling spacer from causing undesired contact friction between wall surfaces of the screw grooves and the circulation passage, or even exerting a normal pressure against the walls thereof, the outer diameter of the cylinder is made smaller than that of the ball.
In the present invention, in order to prevent the spacer from contacting the side wall surface of abruptly curved circulation passage of a small curvature, or even produce a normal pressure to the wall surfaces so as to further exacerbate the increase of frictional resistance, the silhouette of the cylindrical shaped spacer, as observed in the front view, is formed into two inwardly concaved arcuate figures. By so, the probability of frictional contact between surfaces of the cylindrical shaped spacer and the circulation passage wall is reduced to a minimum.
Since the balls contact the spacers with a rather large area, the oil film on the surface of rolling ball is apt to be abrased. In order to minimize the contact surface of the spacer with the ball, the inwardly concaved arcuate surface of the spacer is formed of two truncated conical surfaces as it is observed from the front, and the radius of the arcuate surface is slightly larger than that of the ball, so that the ball makes point contact with two truncated cone surfaces (as it can be observed from the front view.
In the present invention, because the spacer would not rotate in the operation of the ball screw unit, the lubricating oil can not be imparted from ball to ball depending on the spacer balls for media as conventional techniques do. In order to achieve a preferable lubricating effect, a let through hole is provided amidst of the spacer so as to carrying away excessive oil contained in the oil film of the ball and then impart it to adjacent ball with a poorly thin oil film or even in lack of oil at all, therefore, the object of uniform distribution of lubricating oil can be attained. Besides, the environmental pollution caused by overflow of the oil can be avoided and cooling effect can also be carried out by smooth flow of the lubricating oil.
In the present invention, for eliminating the disadvantage that the lubricating oil can not ooze smoothly through the closed contact surface between the ball and the spacer, cold instead, passing in vain via the let through hole, the inwardly concaved end surface of the spacer is further formed into a sinuate shape so as to divide the contact surface between the ball and the spacer into a plurality of intermittent contact points, thereby improving the flow of lubricating oil and lowering frictional resistance.
In the present invention, material for example, plastics, high molecular compounds, reinforced plastics, and ceramic are usable for fabricating low friction and anti-abrasive spacer. As for the fabrication method, forming by plastic ejection is recommendable for simplicity and lowering production cost.
Besides, oil containing materials are recommendable for fabricating the spacer so as to solve the problem of exaggerate consumption of lubricant oil in ball screws.